Temperature compensation method for second derivative spectra of seawater nitrate analyzers

Abstract

The application provides a temperature compensation method for second derivative spectrum of a seawater nitrate analyzer, considers the interference factors of high-concentration ions such as chlorine and bromine in seawater on the measurement of nitrate when the spectrum temperature changes, and only needs to measure the current water sample temperature T when measuring the spectrum of the seawater sample, and the second derivative spectrum structure of the standard seawater at this temperature can be obtained after the compensation formula of the second derivative spectrum of the seawater is brought in, as the temperature compensation. The second derivative spectrum data of the standard seawater is deducted from the second derivative spectrum of the actual water sample, the separation and deduction of the temperature compensation and the spectrum of the high-concentration ions in the seawater can be realized, the influence of the ion spectrum in the seawater on the nitrate spectrum is inhibited, the measurement precision of the nitrate is improved, and the method is suitable for the rapid online analysis of the nitrate in seawater by the spectral method.

Description

TECHNICAL FIELD

[0001] The present application belongs to the technical field of rapid analysis of seawater quality by spectroscopy, and particularly relates to a temperature compensation method for second derivative spectra of a spectroscopic seawater nitrate analyzer. BACKGROUND

[0002] Seawater nitrate is one of the important parameters of marine water quality monitoring and marine ecological environment investigation, and is also one of the indexes for evaluating the eutrophication of a sea area. The nitrate analyzer based on spectroscopy has the characteristics of simple operation, high sensitivity, reagent-free, and suitability for in-situ online detection, and has gradually become an important monitoring instrument. At present, in the nitrate analyzer based on absorption spectroscopy, the ultraviolet absorption spectroscopy method is generally used, that is, a deuterium lamp light source is used to measure the ultraviolet absorbance of the sample by a spectrometer, and the absorbance data near the wavelength of 220 nm is used to implement spectral modeling to realize the measurement of the concentration of nitrate.

[0003] Due to the complex composition of seawater, high concentrations of chlorine and bromine ions are contained, which not only form spectral overlap interference with nitrate, but also the absorption spectrum of high-concentration ions in seawater is easily affected by environmental temperature, and the spectral structure and spectral intensity change with temperature. Therefore, a temperature compensation model of the absorption spectrum of seawater ions needs to be established first, and then the interference components of chlorine and bromine ions in the seawater spectrum can be eliminated to realize the accurate measurement of the concentration of nitrate. In the wavelength range of 210 to 240 nm, the ultraviolet absorbance curve of seawater ions shows a monotonous decreasing trend with the increase of wavelength, and the spectral intensity and spectral structure change with temperature. Therefore, the temperature variation curve of the absorbance of seawater ions can be described by an e-exponential decay curve containing wavelength and temperature parameters, so as to compensate and correct the distortion of the absorbance curve induced by temperature variation.

[0004] Recent studies have shown that although the ultraviolet absorbance can be used to retrieve the content of nitrate in seawater, direct modeling using only the ultraviolet absorbance curve is easily affected by various interference factors in the water body. If the spectral algorithm is improved, the original absorbance curve is first subjected to derivative operation, and then the second derivative spectrum is used for spectral modeling, which can significantly suppress the turbidity scattering interference in the water and improve the measurement accuracy. Compared with the seawater absorbance curve, the second derivative spectrum in the wavelength range of 210 to 240 nm has more complex structure, not only the peaks and valleys appear alternately, but also the peak position moves with the change of environmental temperature. The conventional e-exponential temperature compensation model for the absorbance curve cannot describe the above complex second derivative spectrum structure, and there is no related report on the temperature compensation method for the second derivative spectrum of seawater.

[0005] In summary, it is necessary to design a temperature compensation method for the second derivative spectrum of an optical seawater nitrate analyzer to solve the spectral temperature drift problem in the spectroscopic seawater nitrate analyzer.

[0006] The above information disclosed in the BACKGROUND section is only for increasing the understanding of the background of the present application, and therefore, can include information that does not constitute the prior art known to those of ordinary skill in the art. SUMMARY

[0007] The present application provides a second derivative spectrum temperature compensation method for optical seawater nitrate analyzer and a control method thereof, which can implement temperature compensation for the second derivative spectrum of seawater ultraviolet absorbance curve, improve the environmental adaptability of the spectral analysis instrument, and improve the nitrate measurement accuracy.

[0008] To achieve the above technical purposes, the present application adopts the following technical solutions:

[0009] A second derivative spectrum temperature compensation method for optical seawater nitrate analyzer, comprising the following steps:

[0010] Step one, under the condition that the water sample temperature is T1, measure the ultraviolet absorption spectrum of standard seawater and pure water sample, take pure water as reference, calculate the standard seawater ultraviolet absorbance curve, and record the obtained absorbance values as a sequence in the order of wavelength from small to large:

[0011] {a1, a2, a3, …, a m} (1);

[0012] According to the number of wavelengths of the measured absorption spectrum, a total of m absorbance data are recorded.

[0013] Step two, according to the first derivative calculation formula:

[0014] fd n =a (n+1) -a n (2);

[0015] Using numerical derivative method, the first derivative of the absorbance sequence (1) is calculated one by one, and the first derivative is arranged as a sequence:

[0016] {fd1, fd2, fd3, …, fd (m-1)} (3)。

[0017] Step three, according to the second derivative calculation formula:

[0018]

[0019] Using numerical derivative method, the second derivative of the absorbance sequence (1) is calculated one by one, and the second derivative is arranged as a sequence:

[0020]

[0021] Where T1 subscript describes the data is obtained at temperature T1.

[0022] Step four, change the temperature of the standard seawater sample to T2, T3, Tn, repeat steps one to three to obtain the second derivative sequence at multiple temperature conditions.

[0023] Step five, arrange the obtained n sets of second derivative sequences into a matrix MD according to formula (6):

[0024]

[0025] Where Tn subscript is only used to describe the data arrangement method and does not participate in numerical operation.

[0026] Step six, singular value decomposition operation is performed on the matrix MD as follows:

[0027]

[0028] Where, U matrix is the basic spectrum, S matrix is the singular value diagonal matrix, and V matrix is the intensity change of each basic spectrum. After singular value decomposition operation, each element value in the U matrix can be obtained, which contains (m-2) rows and (m-2) columns, and can be represented as:

[0029]

[0030] Step seven, extract the first two columns S1 and S2 of the U matrix as two temperature-related fingerprint spectra:

[0031]

[0032]

[0033] Step eight, using the temperature-related fingerprint spectra S1 and S2 in step seven, a linear combination method is used to establish a linear equation to reconstruct the second derivative spectrum sequence, which can be represented as:

[0034]

[0035] Where b1 and b2 represent the intensity parameters of the fingerprint spectrum, and are related to temperature. Solving the linear overdetermined equation (11) through spectral reconstruction fitting, the parameter value at temperature T can be obtained.

[0036] Step nine, the second derivative sequence at temperatures T1, T2, …, Tn in step five is brought into the overdetermined equation (11) respectively, and the b1 and b2 parameter values are solved. For each temperature T, one equation can be established, and after solving, one set of b1 and b2 parameters can be obtained. For temperatures T1, T2, …, Tn, n sets of parameters can be obtained. All temperature and temperature parameter data are recorded in sequence as a sequence:

[0037] (T1, T2, T3, …, T n} (12);

[0038]

[0039]

[0040] Step ten, the polynomial function of the parameter b1, b2 sequence and temperature T is established:

[0041] b1(T) = j0 + j1T + j2T 2 (15);

[0042] b2(T) = k0 + k1T + k2T 2 (16);

[0043] Wherein, j, k are temperature fitting coefficients.

[0044] Step eleven, the temperature sequence (12) and the temperature parameter sequence (13), (14) are brought into formula (15), (16), and the least square method is used for fitting to obtain the coefficients j0, j1, j2 and k0, k1, k2.

[0045] Step twelve, the temperature related fingerprint spectrum S1, S2 in step seven and the coefficients obtained in step eleven are used to establish the compensation spectrum formula of the second derivative spectrum of seawater at any temperature:

[0046]

[0047] Step thirteen, the temperature and the second derivative spectrum of the actual water sample are measured, the second derivative temperature compensation number spectrum (17) is deducted from the second derivative spectrum of the actual water sample, the temperature compensation is implemented, and the second derivative spectrum after the temperature compensation is used for nitrate detection.

[0048] Compared with the prior art, the advantages and positive effects of the present application are that: the second derivative spectrum temperature compensation method of the present application for the optical method seawater nitrate analyzer considers the interference factors of high concentration ions such as chlorine and bromine in seawater on the measurement of nitrate when the spectrum temperature changes, only needs to measure the current water sample temperature T when measuring the spectrum of the seawater sample, and can obtain the second derivative spectrum structure of the standard seawater at this temperature after being brought into the compensation formula of the second derivative spectrum of seawater, as temperature compensation. The second derivative spectrum data of the standard seawater is deducted from the second derivative spectrum of the actual water sample, the temperature compensation and separation deduction of the spectrum of high concentration ions in seawater can be realized, the influence of the ion spectrum in seawater on the nitrate spectrum is suppressed, the measurement precision of nitrate is improved, and the present application is suitable for the rapid online analysis of seawater nitrate by the spectral method. BRIEF DESCRIPTION OF DRAWINGS

[0049] In order to more clearly illustrate the technical solutions in the embodiments of the present application, the drawings needed to be used in the embodiments will be briefly introduced as follows. Obviously, the drawings in the following description are some embodiments of the present application, and other drawings can be obtained by those skilled in the art without any creative effort based on these drawings.

[0050] Figure 1 The figure is a spectrum diagram of the temperature-dependent fingerprint spectrum S1, S2.

[0051] Figure 2 The figure is a spectrum diagram of the second derivative spectrum of the standard seawater at 10℃ and the reconstructed spectrum obtained by fitting S1 and S2.

[0052] Figure 3 The figure is a function relationship diagram of the intensity parameters b1, b2 of the fingerprint spectrum and the temperature T. DETAILED DESCRIPTION

[0053] The preferred embodiments of the present application will be described below with reference to the drawings. Those skilled in the art should understand that these embodiments are only used to explain the technical principles of the present application, and are not intended to limit the protection scope of the present application.

[0054] It should be noted that in the description of the present application, the terms indicating the direction or positional relationship of the terms "center", "upper", "lower", "left", "right", "vertical", "horizontal", "inner", "outer" and the like are based on the direction or positional relationship shown in the drawings, which is only for the convenience of description, and does not indicate or imply that the device or element must have a particular orientation, be constructed and operated in a particular orientation, and therefore cannot be understood as a limitation on the present application. In addition, the terms "first", "second", "third" are only for the purpose of description, and cannot be understood as indicating or implying relative importance.

[0055] In order to make the objects, technical solutions and advantages of the embodiments of the present application clearer, the technical solutions in the embodiments of the present application will be described clearly and completely below with reference to the drawings in the embodiments of the present application.

[0056] A temperature compensation method for the second derivative spectrum of an optical seawater nitrate analyzer, comprising the following steps:

[0057] Step one, under the condition that the water sample temperature is T1, the ultraviolet absorption spectrum of the standard seawater and pure water sample is measured, the pure water is taken as the reference, the standard seawater ultraviolet absorbance curve is calculated, and the obtained absorbance values are recorded as a sequence in the order of wavelength from small to large:

[0058] {a1, a2, a3, …, a m} (1);

[0059] According to the number of wavelengths of measuring absorption spectrum, m absorbance data are recorded.

[0060] Step two, according to the first derivative calculation formula:

[0061] fd n = a (n+1) -a n (2) ;

[0062] Using numerical derivative method, the first derivative of absorbance series (1) is calculated one by one, and the first derivative is arranged into series:

[0063] {fd1,fd2,fd3,……,fd (m-1)} (3).

[0064] Step three, according to the second derivative calculation formula:

[0065]

[0066] Using numerical derivative method, the second derivative of absorbance series (1) is calculated one by one, and the second derivative is arranged into series:

[0067]

[0068] Where T1 subscript describes that the data is obtained at temperature T1.

[0069] Step four, change the temperature of standard seawater sample to T2, T3, …, T n , repeat steps one to three to obtain the second derivative series at multiple temperature conditions.

[0070] Step five, arrange the obtained n groups of second derivative series into matrix MD according to formula (6):

[0071]

[0072] Where Tn subscript is only used to describe the data arrangement method and does not participate in numerical operation.

[0073] Step six, singular value decomposition operation is performed on matrix MD as follows:

[0074]

[0075] Where U matrix is the basic spectrum, S matrix is the singular value diagonal matrix, and V matrix is the intensity change of each basic spectrum. After singular value decomposition operation, the value of each element in U matrix can be obtained, which contains (m-2) rows and (m-2) columns, and can be represented as:

[0076]

[0077] Step seven, extract the first two columns S1 and S2 of U matrix as two temperature-dependent fingerprint spectra:

[0078]

[0079]

[0080] Step eight, using the temperature-dependent fingerprint spectra S1 and S2 in step seven, a linear combination method is used to establish a linear equation to reconstruct the second-order derivative sequence, which can be expressed as:

[0081]

[0082] where b1 and b2 represent the intensity parameters of the fingerprint spectrum, and are related to temperature. Solving the linear overdetermined equation (11), through spectral reconstruction fitting, the parameter value at temperature T can be obtained.

[0083] Step nine, bring the second derivative sequence at temperatures T1, T2, …, Tn in step five into the overdetermined equation (11) respectively, and solve the b1 and b2 parameter values. For each temperature T, one equation can be established, and after solving, a set of b1 and b2 parameters can be obtained. For temperatures T1, T2, …, Tn, n sets of parameters can be obtained. Record all temperature and temperature parameter data in sequence as a sequence:

[0084] {T1, T2, T3, …, T n} (12);

[0085]

[0086]

[0087] Step ten, establish a polynomial function of the parameter b1, b2 sequence and temperature T:

[0088] b1(T) = j0 + j1T + j2T 2 (15);

[0089] b2(T) = k0 + k1T + k2T 2 (16);

[0090] where j and k are temperature fitting coefficients.

[0091] Step eleven, bring the temperature sequence (12) and the temperature parameter sequences (13), (14) into formulas (15), (16), and use the least squares method to fit to obtain coefficients j0, j1, j2 and k0, k1, k2.

[0092] Step 12, using the temperature related fingerprint spectrum S1, S2 in step 7 and the coefficient obtained in step 11, the compensation spectrum formula of the second derivative spectrum of seawater at any temperature is established:

[0093]

[0094] Step 13, measure the temperature and second derivative spectrum of the actual water sample, and subtract the second derivative temperature compensation spectrum (17) from the second derivative spectrum of the actual water sample to implement temperature compensation, and use the temperature compensated second derivative spectrum for nitrate detection.

[0095] The following is described with a specific example:

[0096] Step 1, take pure water as reference, measure the absorbance curve of the standard seawater (salinity 35) sample at 5℃ sample temperature, and calculate the second derivative spectrum, which includes the following steps:

[0097] 1) measure the ultraviolet absorption spectrum of pure water sample by using deuterium lamp light source and optical fiber spectrometer, the optical fiber spectrometer can measure the light intensity of each pixel, and record the transmission light intensity of pure water as a sequence in the order of wavelength from small to large:

[0098]

[0099] The number of elements in the sequence is consistent with the number of pixels of the spectrometer, and in this embodiment, a total of 1024 light intensity data are recorded. The wavelength values of each pixel are recorded in the order from small to large:

[0100] {λ1,λ2,λ3,……,λ 1024} (2).

[0101] 2) measure the ultraviolet absorption spectrum of the standard seawater sample with salinity 35, and record the transmission light intensity as a sequence in the order of wavelength from small to large:

[0102] {I1,I2,I3,……,I 1024} (3).

[0103] 3) take pure water as reference, calculate the absorbance at each wavelength according to the absorbance formula:

[0104]

[0105] record the seawater absorbance data as a sequence in the order of wavelength from small to large:

[0106] {a1,a2,a3,……,a 1024} (5).

[0107] 4) The wavelength data corresponding to each pixel in the spectrometer usually presents a uniform linear distribution, i.e. the difference between adjacent wavelength data in the wavelength sequence is fixed and unchanged, so the wavelength data can be ignored when calculating the derivative spectrum, and the first derivative calculation formula can be written as:

[0108] fd n = a (n+1) -a n (6).

[0109] Using the numerical derivative method, the first derivative of the absorbance sequence (5) is calculated one by one, and the first derivative is arranged into a sequence:

[0110] {fd1,fd2,fd3,……,fd (1024-1)} (7) ;

[0111] A total of 1023 first derivative data are recorded.

[0112] 5) According to the second derivative calculation formula:

[0113]

[0114] Using the numerical derivative method, the second derivative of the absorbance sequence (5) is calculated one by one. The second derivative is arranged into a sequence in order of increasing wavelength as:

[0115]

[0116] where the superscript indicates that the current water sample temperature is 5℃, and a total of 1022 second derivative data are recorded.

[0117] Step two, change the sample temperature, measure the absorbance curve of the standard seawater sample, which includes the following steps:

[0118] 1) In this embodiment, the temperature of the standard seawater sample is increased from 5℃ to 30℃, and each time the temperature is increased by 5℃, step one is repeated, and a total of six groups of second derivative sequences are obtained.

[0119] 2) The six groups of second derivative sequences obtained are arranged into a matrix MD:

[0120]

[0121] Each column in the MD matrix represents a group of second derivative sequences under a temperature condition, and a total of six groups of temperature data are included.

[0122] Step three, obtain the temperature-related fingerprint spectrum, which includes the following steps:

[0123] 1) Perform singular value decomposition operation on the matrix MD as follows:

[0124]

[0125] where U is the basic spectrum, S is the diagonal matrix of singular values, and V is the intensity variation of each basic spectrum. After singular value decomposition operation, each element in the obtained U matrix, which contains 1022 rows and 1022 columns, can be expressed as:

[0126]

[0127] 2) Extract the first two columns S1 and S2 of the U matrix as the temperature-dependent fingerprint spectrum:

[0128]

[0129]

[0130] The wavelength is taken as the abscissa, and S1 and S2 are shown as follows: Figure 1

[0131] Step four, using the temperature-dependent fingerprint spectrum, the second-order spectrum derivative at each temperature is implemented for spectral reconstruction, which includes the following steps:

[0132] 1) Using the temperature-dependent fingerprint spectrum S1 and S2 in step three, a linear combination method is used to establish a linear equation to reconstruct the second-order spectrum derivative sequence, which can be expressed as:

[0133]

[0134] where b1 and b2 are intensity parameters of the fingerprint spectrum, and are related to temperature. By solving the overdetermined equation (15), spectral reconstruction fitting can be achieved, and the parameter value at temperature T is obtained.

[0135] 2) The second-order derivative sequence in step two is brought into the overdetermined equation (15) respectively, and the b1 and b2 parameter values are solved respectively. For each temperature T, the corresponding second-order derivative sequence is substituted, and one equation is established. After solving, a set of b1 and b2 parameters can be obtained. As shown in the following figure, the second-order derivative spectrum of the standard seawater at 10°C and the reconstructed spectrum obtained by fitting S1 and S2. In this embodiment, corresponding to the water sample temperature 5°C, 10°C, …, 30°C, 6 sets of parameters can be obtained, and all temperature and temperature parameter data are recorded as a sequence: Figure 2 where the temperature sequence is recorded as:

[0136] {5, 10, …, 30} (16);

[0137] The fitted parameters are recorded in the sequence according to the temperature order:

[0138]

[0139] ​​

[0140]

[0141] Each number series contains 6 data corresponding to 5-30℃.

[0142] Step five, establish the quadratic polynomial formula of temperature parameter and temperature T, obtain the temperature fitting coefficients, specifically including the following steps:

[0143] 1) Establish the polynomial function relationship of parameters b1, b2 and temperature T:

[0144] b1(T)=j0+j1T+j2T 2 (19);

[0145] b2(T)=k0+k1T+k2T 2 (20);

[0146] Wherein, j, k are temperature fitting coefficients.

[0147] 2) Bring the temperature series (16) and the temperature parameter series (17), (18) into formula (19) and (20) respectively, and fit by using the least square method to obtain coefficients j0, j1, j2 and k0, k1, k2. The coefficients obtained in this embodiment are:

[0148] j0=-0.0206782;j1=-0.0005379;j2=0.0000104;

[0149] k0=-0.0105066;k1=0.0007879;k2=-0.0000056;

[0150] The function relationship of parameters b1, b2 and temperature T is shown in Figure 3 .

[0151] Step six, establish the seawater second-order derivative spectrum temperature compensation formula by using the temperature related fingerprint spectrum S1, S2, and implement temperature compensation for the seawater second-order derivative spectrum, specifically including the following steps:

[0152] 1) Collect seawater sample, measure the water sample temperature T. According to step one, continue to measure the second-order derivative spectrum of the seawater sample at this temperature {sd1, sd2, sd3, …, sd 1022}(21);

[0153] 2) Use the second-order spectrum compensation formula:

[0154]

[0155] Bring in the current temperature value T, calculate the standard seawater second-order derivative spectrum temperature compensation spectrum series.

[0156] 3) The second derivative spectrum of the seawater sample is subtracted from the corresponding elements in the temperature compensation sequence, and the temperature-compensated second derivative spectrum of the seawater sample is obtained:

[0157] {sd1-cp1,sd2-cp2,sd3-cp3,……,sd 1022 -cp 1022} (23);

[0158] The temperature-compensated second derivative spectrum of the seawater sample can be used for nitrate spectral modeling and concentration measurement. The second derivative spectrum temperature compensation method of the present application has the advantages of fast detection speed and easy implementation, and can realize temperature compensation and separation of high-concentration ion spectra in seawater, suppress the influence of ion spectra in seawater on nitrate spectra, improve the measurement accuracy of nitrate, and is suitable for rapid online analysis of seawater nitrate by spectral method.

[0159] The above examples are only used to illustrate the technical solutions of the present application, but not to limit it; although the present application has been described in detail with reference to the foregoing examples, the technical solutions recorded in the foregoing examples can still be modified by those of ordinary skill in the art, or some technical features can be replaced by equivalents; and these modifications or replacements do not make the essence of the corresponding technical solutions deviate from the spirit and scope of the technical solutions claimed by the present application.

Claims

1. A method for temperature compensation of second derivative spectra of seawater nitrate analyzer, comprising the following steps: Step 1, measure the ultraviolet absorption spectra of standard seawater and pure water samples at the temperature of T1, take pure water as reference, calculate the ultraviolet absorbance curve of standard seawater, and record the absorbance curve as a sequence; Step 2, calculate the first derivative of the absorbance curve and record it as a first derivative sequence; Step 3, calculate the second derivative of the absorbance curve and record it as a second derivative sequence; Step 4, repeat steps 1-3 to obtain the second derivative sequence at various temperatures of T2, T3, …, Tn; Step 5, record all the obtained second derivative sequences as a matrix; Step 6, perform singular value decomposition operation on the matrix; Step 7, extract the first 2 columns of the U matrix obtained by singular value decomposition as the temperature-dependent fingerprint spectrum; Step 8, use the temperature-dependent fingerprint spectrum to establish a linear equation by linear combination method to reconstruct the second derivative sequence and solve the intensity parameters of the fingerprint spectrum; Step 9, bring the second derivative sequences at temperatures of T1, T2, …, Tn into the linear equation respectively to solve the intensity parameters of the temperature-dependent fingerprint spectrum respectively and record them as a sequence of temperature-dependent fingerprint intensity parameters; Step 10, establish a polynomial function of the intensity parameters of the fingerprint spectrum and temperature T; Step 11, obtain multiple coefficients of the polynomial function in step 10 by least square fitting; Step 12, obtain the compensation formula of seawater second derivative spectrum at any temperature by using the temperature-dependent fingerprint spectrum in step 7 and the coefficients obtained in step 11; Step 13, measure the actual water sample temperature and second derivative spectrum, deduct the second derivative temperature compensation spectrum from the actual water sample second derivative spectrum to implement temperature compensation, and use the temperature-compensated second derivative spectrum for nitrate detection.

2. The method for temperature compensation of second derivative spectra for seawater nitrate analyzers according to claim 1, characterized in that, The method for recording the absorbance curve as a sequence is to record the absorbance of standard seawater as a sequence in the order of wavelength from small to large: {a1,a2,a3,……,a m}.

3. The method for temperature compensation of second derivative spectra for seawater nitrate analyzers according to claim 2, characterized in that, The calculation method of each element in the first derivative sequence is: fd n = a (n+1) - a n ; The recording method of the first derivative sequence is: {fd1,fd2,fd3,……,fd (m-1)} The calculation method of each element in the second derivative sequence is: The recording method of the second derivative sequence is: Wherein, the subscript T1 describes the data obtained at temperature T1. 4.The method for temperature compensation of second derivative spectra of seawater nitrate analyzer according to claim 3, characterized in that: The absorbance is measured and the second derivative sequence is calculated at the temperatures of T1, T2, …, Tn of the standard seawater sample, and the arrangement method of the matrix is: Wherein, the subscript Tn is used to describe the data arrangement method and does not participate in numerical operation.

5. The method for temperature compensation of second derivative spectra for seawater nitrate analyzers according to claim 4, characterized in that: The singular value decomposition operation is: The U matrix is a basic spectrum, which can be expressed as: The temperature-dependent fingerprint spectrum is:

6. The method for temperature compensation of second derivative spectra for seawater nitrate analyzers according to claim 5, characterized in that, The linear equation is: The intensity parameters of the fingerprint spectrum are b1 and b2.

7. The method for temperature compensation of second derivative spectra for seawater nitrate analyzers according to claim 6, characterized in that, The second derivative sequences at temperatures of T1, T2, …, Tn are a column of numerical data corresponding to the temperature in the matrix MD.

8. The method for temperature compensation of second derivative spectra for seawater nitrate analyzers according to claim 6, characterized in that, For each temperature T, a set of b1, b2 parameters can be solved, and the method for recording the temperature-dependent fingerprint spectrum intensity parameter sequence is to record all the temperature and b1, b2 parameter data in sequence as a sequence: { T1, T2, T3,..., T n}; 9. The temperature compensation method for the second derivative spectrum of the seawater nitrate analyzer according to claim 8, characterized in that, The polynomial function specifically adopts the following formula: b1(T) = j0 + j1T + j2T2 2 ; b2(T) = k0+ k1T + k2T2 2 ; The coefficients of the polynomial function are j0, j1, j2 and k0, k1, k2.

10. The control method for the temperature compensation method for the second derivative spectrum of the seawater nitrate analyzer according to claim 9, characterized in that, The compensation formula of the seawater second derivative spectrum specifically adopts the following formula:

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